Sidebar Sidebar
  • You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
  • Click here to find out how to win free radios from Retevis!

A Receive Pre-Amp you can build!

Handy Andy said:
KEWL!

Thought I'd help a bit here...

View attachment 24198

It's (the above) is preliminary - no "Mask" for the traces yet.

But managed to glean this from your schematic and parts list - thank you.

Looks like something that can be "inserted" directly - the "13.8V" deal is just that - power supply. Anything over 6 volts the thing worked fine. So this works for any RX/TX switch radio and you simply soldered this ahead of the dropping resistor for the RX LED - so it "toggled on or off" with the RX side of things - took like 50mA at the most.

As so made, the design was for INSERTION across the leads of a relay - like those used in Amps that did not come with a built-in pre amp. So TSW2 was INPUT (Signal In) FROM your antenna and TSW1 was the OUTPUT (Signal Out) to the radios' coax connector.

Sidebar SEGUE -
Note the Orientation of Q1 - Original part outline was for a BCE PINOUT - but the 1675 used has a PINOUT of ECB - so you know.​

The 13.8 thingy was to have been set up to use the POWER side of the Relays unused contacts (Assuming when you're RX-ing the amp was off) when the Amps' Relays contacts were at idle - so it received Power from the amp - but only when the amp was NOT engaged, when the AMP kicked in - the relay broke the power feed and switched it to the amps own internal feeds to the amplifier - turning off this device. (Basically a DPDT style amp - one side for antenna switching the other for power switching.) So you have to locate the amps bypass RX cap for when the amps not on, and this is where TSW1 and TSW2 went.

The TSW1 and TSW2 - note the diode used ... helped keep the diode conducting into Q1's BASE lead, keeping it and Q1 from self destruct (NPN) so that went across the RELAY terminals that had the antenna set up to BYPASS the amp when it's not engaged.

These were offered as kits on an older 2290-based amp that people could buy and install in a Grant, Cobra 148 or even their RCI2950's as a helper - Modulator. They were the predecessor to the RFX MOSFET kits you'd see today. These boards were the pre-amps, designed for them and came as a separate kit from the supplier - either RF Limited or CB City had offered these as accessories.

Fun little device, brings back TONS of memories - and an Old Grant I once had that the XYL never returned.

SIGH.

:+> Andy <+:
Click to expand...

The transistor is in backwards. I build these and sell these and there are some 1675 pinouts that are switched. I bought some 1675 on ebay and the pins were different from a batch I bought 2 months earlier. But the 1675 is in backwards. Also check the big brown cap. It blows easily if powered wrong.
Usually the green in input and yellow is out. So the green would go closest to the antenna input and yellow out. Good luck
 
  • Like
Reactions: tecnicoloco and Shadetree Mechanic
Handy Andy said:
KEWL!

Thought I'd help a bit here...

View attachment 24198

It's (the above) is preliminary - no "Mask" for the traces yet.

But managed to glean this from your schematic and parts list - thank you.

Looks like something that can be "inserted" directly - the "13.8V" deal is just that - power supply. Anything over 6 volts the thing worked fine. So this works for any RX/TX switch radio and you simply soldered this ahead of the dropping resistor for the RX LED - so it "toggled on or off" with the RX side of things - took like 50mA at the most.

As so made, the design was for INSERTION across the leads of a relay - like those used in Amps that did not come with a built-in pre amp. So TSW2 was INPUT (Signal In) FROM your antenna and TSW1 was the OUTPUT (Signal Out) to the radios' coax connector.

Sidebar SEGUE -
Note the Orientation of Q1 - Original part outline was for a BCE PINOUT - but the 1675 used has a PINOUT of ECB - so you know.​

The 13.8 thingy was to have been set up to use the POWER side of the Relays unused contacts (Assuming when you're RX-ing the amp was off) when the Amps' Relays contacts were at idle - so it received Power from the amp - but only when the amp was NOT engaged, when the AMP kicked in - the relay broke the power feed and switched it to the amps own internal feeds to the amplifier - turning off this device. (Basically a DPDT style amp - one side for antenna switching the other for power switching.) So you have to locate the amps bypass RX cap for when the amps not on, and this is where TSW1 and TSW2 went.

The TSW1 and TSW2 - note the diode used ... helped keep the diode conducting into Q1's BASE lead, keeping it and Q1 from self destruct (NPN) so that went across the RELAY terminals that had the antenna set up to BYPASS the amp when it's not engaged.

These were offered as kits on an older 2290-based amp that people could buy and install in a Grant, Cobra 148 or even their RCI2950's as a helper - Modulator. They were the predecessor to the RFX MOSFET kits you'd see today. These boards were the pre-amps, designed for them and came as a separate kit from the supplier - either RF Limited or CB City had offered these as accessories.

Fun little device, brings back TONS of memories - and an Old Grant I once had that the XYL never returned.

SIGH.

:+> Andy <+:
Click to expand...
I realize this is a couple of years out - but - R2 & R3 are reversed in the original drawing. That makes the, very nice BTW, "engineered" pic/schematic incorrect. On the circuit board, R3 should be on the top right and R2 the resistor below it. Then to clarify - R2 is 470 ohms, and R3 is 270 ohms. (270 top right, 470 below it.) I have one of these boards, exactly as in the original pictures, and did not know its purpose until I stumbled across this post. Mine came out of a Cobra 29 LTD Classic and C8 had been removed to be replaced by TSW1, TSW2. Thank you for letting me know the board's purpose.
 
  • Like
Reactions: tecnicoloco, Naysayer, 357magnum and 1 other person
thanks for the reply. I have a bunch of these and will try a ceramic cap.

Was just on ebay looking for these silver mica caps they are crazy priced $32.00 for 2 and other crazy prices for other silver mica caps. Why such a high price?
 
Last edited:
  • Like
Reactions: Naysayer
Disc ceramic types come in multiple types. Different ceramic dielectrics will have their own temperature characteristics.

Best ceramic disc to replace a silver mica is type "NPO". This one is more temperature stable.

Silver mica's claim to fame was low internal resistance, since silver has the lowest resistivity of all the metals on the periodic table. Some folks claim that they are more temperature stable, but that's only partly true. As a rule, a circuit with any real power in it benefits from using a silver mica because it won't heat up as much from the AC current that it carries. A preamp is a pretty low-power circuit, so any capacitor that's fairly stable should work equally well.

73
 
  • Like
Reactions: Naysayer
nomadradio said:
Disc ceramic types come in multiple types. Different ceramic dielectrics will have their own temperature characteristics.

Best ceramic disc to replace a silver mica is type "NPO". This one is more temperature stable.

Silver mica's claim to fame was low internal resistance, since silver has the lowest resistivity of all the metals on the periodic table. Some folks claim that they are more temperature stable, but that's only partly true. As a rule, a circuit with any real power in it benefits from using a silver mica because it won't heat up as much from the AC current that it carries. A preamp is a pretty low-power circuit, so any capacitor that's fairly stable should work equally well.

73
Click to expand...
Thanks nomadradio for the info on the silver mica caps
 
  • Like
Reactions: Naysayer

dxChat
Help Users
  • No one is chatting at the moment.


      Top Bottom
      Back